Oric Atmos diag - minimum chips required?

[john]

So I've gone down the rabbit hole of building my own oric atmos clone and after assembling everything it shows no signs of life.
5V is all good, 1MHz clock in phi in and out on CPU are good but data/address lines are not showing any life.

Before pulling out the schematic and scope I have a question around the oric atmos diag ROM by Mike Brown.

I'm fairly sure the ULA is working (Or at least it was the last time I used it in a Oric which I don't have anymore) but not sure about 6522/RAM.
6502 also confirmed working - tested in a BBC

If I go back to basics and pull all the chips, what are the minimum chips required to fire up the diag rom to at least see some video?

6502, ULA for sure, but what about the rest?
6522/LM386/8912 are needed for sound but will I see any video without these?
Is the RAM also needed?

[Sodiumlightbaby]

To get video out you need ROM, 6502, ULA and DRAM (and glue logic) working. But with just ROM and 6502 you will get codes put on the bus /expansion port). I highly recommend studying the diag ROM documentation and follow the bring-up step by step.

[Dbug]

Could also be the CPU not starting properly due to some issue with the RESET circuitry.

[john]

Thanks for the tips.

Success - it's booting!

A few things I found wrong.

Chips required for diag rom are RAM, ROM, CPU, ULA, logic chips. (For some reason I thought RAM wasn't need to boot into the ROM but that wasn't my main issue. Even without chips there was no video)

I started to do continuity testing around the RBG output and didn't see anything on the DIN5 connector.
See if you can spot the issue

20240704_212712.jpg

When soldering in the DIN5 connector the footprint of the connector I had must be larger than the original used so it covered up RP2. Without RP2 there is no 220R resistor array connecting to the video out. (It's supposed to be between IC22 and the DIN5 connector.)

I soldered SMD resistors to the bottom side of the PCB and now I have video. Must be the only Oric in the world with a SMD part

Next with video out it still wasn't booting. Tried shorting reset to ground and after that it booted! Hurrah!!!!

The schematic says 2k2 in series with a 1uF Cap but it's not happy with this. Changed it to 10uF cap and it's booting 100% of the time now from power up.

Now just the sound to sort out but I think that's a suspect 8912 - but that's the easy bit to sort.

[Sodiumlightbaby]

Great progress! Looks like you are missing blue on the video though? You may want to make sure you got the 6522 VIA passing too as it sits in front of the 8912.

[john]

Thanks, forgot about the blue. Thought it looked off. Bad solder joint on the SMD. Got blue now as well.

VIA also passes

[Sodiumlightbaby]

How does the 8912 test go? Does the keyboard pass?
The diag ROM documentation has very helpful list of things to test in order if the 8912 doesn't pass.

[john]

I don't have the loopback connectors so the diag test halts when testing the VIA PORTA

Keyboard is working.

A quick google, and tried SHOOT.
Sound is working too

[ibisum]

Great work! How exciting - a brand new Oric!

Got the dflat ROM? That'd probably be fun as well.